Creeper - Has repeatedly blown me to hell and back when I least expected it. Fuck these things.
Spider - A bit annoying. As I have a fully enclosed glass house though, they're not much of a threat - and now with my added glass spikes, they can't climb up and will invariably end up dying a horrible slow death in my water traps if they see me and run for me.
Skeleton - I HATE their arrows. SO. ANNOYING. However, they rarely kill me - I often kill them by luring them onto land when it's daylight, they never harm me and I don't waste any weaponry.
Zombie - Easy to kill. Makes the most unnerving noises, but it's never done serious harm to me.
Pig - Um, yeah. I traditionally kill these guys with feathers. TICKLE THEM TO DEATH
Ghasts - Never encountered.
Spider Jockey - Never encountered.
Overall, creepers, out of all the ones I've seen. So irritating. At least I now have a bow and arrow to deal with them if they chase me.